This invention relates to a cooling system for rejecting waste heat. In more detail, the invention relates to a cooling system for rejecting waste heat from a thermal-electric power plant incorporating a cooling tower adapted to dry operation under normal ambient temperature conditions but including combination cooling capability for use on hot summer days.
As the world demand for electrical power increases, more and larger thermal-electric power plants are being built to meet this need. Of these plants, even the most efficient are capable of converting only about 40% of their heat input into electricity. The remaining 60% of this heat is wasted and must be expelled to the environment. This has usually been accomplished by circulating a large flow of water from a natural source such as a river, lake, or sea, through the plant's steam condenser, and then returning the water to its source after its temperature has been raised by the hot condensing steam. The wisdom of this procedure has been opened to question due to environmental and ecological problems stemming from the temperature rise caused in the natural source.
To avoid this "thermal pollution" of natural bodies of water, alternative methods of cooling power plants have been devised. These include man-made cooling ponds and lakes, spray ponds and spray canals, evaporative cooling towers, and dry cooling towers. Man-made cooling ponds and lakes function similarly to their natural counterparts. Spray ponds and canals and evaporative cooling towers function by flowing water through the plant steam condenser and then cooling the heated water back down to its original temperature by causing a sufficiently large portion of the flow to evaporate, carrying the waste heat into the atmosphere. The cooled water is then recirculated through the plant condenser. All of these wet systems consume large quantities of water to replace the water that is evaporated into the air.
In dry cooling tower systems, the water does not come into contact with the air, and thus does not evaporate. Instead it flows through the inside of the tubes of a large heat exchanger (dry cooling tower) and transmits its thermal energy through the tube walls to a stream of air that is caused to flow over the outside of the tubes (similar to the familiar automobile radiator). Because the system is closed to the atmosphere, fluids other than water may be used to carry the thermal energy from the plant condenser to the cooling tower. Studies have shown that it may be economically favorable to use ammonia instead of water in dry cooling systems. In such systems, liquid ammonia would be vaporized by the hot condensing plant steam and would then be transported as a vapor to the cooling tower where it would be condensed back to a liquid by the cool air flowing through the tower.
Both wet (evaporative) and dry cooling tower schemes have their own definite advantages and disadvantages. As already mentioned, dry cooling towers have the advantage that cooling water is not evaporated into the atmosphere, so that the consumptive use of water is negligible. This advantage would be particularly important in arid areas where water may be too scarce to support an evaporative system, or in locations where large quantities of water evaporated into the atmosphere might cause fog and ice which could be a safety hazard as well as environmentally and aesthetically objectionable.
The major drawback to dry cooling systems is their inability to reject heat to the atmosphere as cheaply and efficiently as wet systems, particularly on hot summer days when power demands in many countries (such as the United States) are likely to be highest and plant cooling capacity is most needed.
To best make use of the advantages of both wet and dry systems, a combination cooling system is commonly used that incorporates the high heat rejection potential of evaporative systems, yet does not result in the high evaporative losses and other attendant problems of totally wet systems. Even in areas where water resources are scarce, the heat rejection capability of wet cooling is so superior to that of dry cooling that there are strong incentives to augment dry cooling towers through evaporative cooling on hot days using any water that may be available at the plant site. By such use of combined dry-wet cooling systems, the plant performance can be significantly improved at the price of only a relatively small consumptive use of the available water resource as compared to usage of wet cooling only.
Several methods have been devised to combine dry and wet cooling systems. These currently include (1) separate dry and wet towers, (2) integrated dry and wet towers, (3) dry tower-cooling pond arrangements, and (4) deluge water augmented dry towers. A brief description of each of these systems follows.
1. Separate Dry and Wet Tower
This system simply employs a wet tower along with a separate and distinct dry tower.
2. Integrated Dry and Wet Tower
In an integrated system, the wet tower portion and dry tower portion are physically contained within the same tower structure. The water flow sequence can be the same as for separate dry and wet towers.
3. Dry Tower-Cooling Pond Arrangements
This system is similar to the separate dry and wet tower system, except that a cooling pond replaces the wet tower.
4. Deluge Water Augmented Dry Tower
In this system the flow from the plant condenser passes through a dry tower only. In hot weather the heat rejection capability of the dry tower is increased by deluging or spraying water over the outside of the tower heat exchanger and allowing some of it to evaporate into the air stream.
In addition, in light of the fact that dry cooling systems using ammonia are projected to be less expensive than dry cooling systems using water, only the deluge water augmented dry tower or a separate condenser loop system would have the capability of combining the advantages of wet cooling with that of the less expensive ammonia dry system. The deluge augmentation system is, however, the only currently viable choice for use with an ammonia system, since a special expensive condenser is required in the separate condenser loop system.
There are, however, some major drawbacks to deluge augmented dry tower systems. Some of these are:
1. Buildup of scale on the tower finned cooling surfaces caused by the evaporation of water from these surfaces may seriously degrade the heat rejection performance of the dry tower and requires expensive maintenance and downtime. This method also requires extensive treatment of the delugeate water to reduce the rate of scale buildup.
2. Deluging the outside of the tower heat exchanger with water may significantly cut down the heated surface area with which the air can come in contact. The degree to which this would occur would depend upon the type of heat exchanger surface employed in the dry tower, but for some types of surfaces this effect may actually degrade the performance of the tower rather than augment it.
3. Heat exchanger surfaces which may be most economical for dry tower application may not be suitable for augmentation. This is a complex relationship with many interdependent factors which affect the economics of the cooling tower.
4. Moisture on the outside of the heat exchanger may contribute significantly to its rate of corrosion, leading to early replacement of the cooling surface.
5. Dimensions of the heat exchanger and application of augmentation water must be carefully controlled so as to keep the entire surface wet while operating to inhibit excessive scaling and at the same time prevent excessive holdup of the water at the top of the heat exchanger which would block the flow of air to heat transfer surfaces in that portion.